Subscriber line interface circuits are typically found in the central office exchange of a telecommunications network. A subscriber line interface circuit (SLIC) provides a communications interface between the digital switching network of a central office and an analog subscriber line. The subscriber line comprises a tip line and a ring line. The analog subscriber line connects the SLIC to a subscriber station or subscriber equipment such as a telephone at a location remote from the central office exchange.
In conjunction with the subscriber equipment, the tip and ring lines form a subscriber loop. The SLIC communicates both data and control signals with the subscriber equipment. Control signals tend to require significantly greater voltages and currents than data signals (i.e., voiceband data) on the subscriber loop.
One subscriber equipment control signal requiring relatively high voltages and current is a ringing signal. Typically, the SLIC provides a sinusoidal or trapezoidal ringing signal to the subscriber equipment. Various approaches to ringing signal generation include unbalanced and balanced ringing.
Unbalanced ringing is accomplished by holding one of the tip and ring lines at a pre-determined voltage level (e.g., ground) while providing the other line with the ringing signal. One disadvantage of unbalanced ringing is that the maximum DC operating voltage for the SLIC must be the greater of: i) any DC offset plus the peak ringing voltage, or ii) the peak-to-peak ringing voltage. Such voltage levels tend to limit fabrication of a ringing signal generator to either high voltage integrated circuits or discrete components.
Balanced ringing provides a ringing signal to each of the tip and ring lines. Typically the AC component of the ringing signal applied to both lines is identical with the exception of a 180° phase shift. As a result, balanced ringing requires a significantly lower DC operating voltage than unbalanced ringing. One disadvantage of traditional balanced ringing is that AC matched ringing signals with a relative phase shift of 180° must be generated.